ISC2 Certified Secure Software Lifecycle Professional (CSSLP) Practice Question

While reviewing the design for an IoT gateway that communicates with a cloud service over TCP, you notice developers have hard-coded a constant send buffer size. To let the protocol itself regulate how much data can be in flight based on the receiver's advertised capacity, which TCP mechanism should they rely on instead?

The Nagle algorithm that combines tiny segments before sending
TCP's urgent pointer for expedited data delivery
The slow-start threshold parameter used for congestion avoidance
The sliding-window flow control field carried in each TCP header

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Answer Description

TCP implements flow control through a sliding-window mechanism. Each ACK from the receiver contains an advertised window field indicating how many additional bytes it is prepared to accept. The sender keeps track of this value and transmits multiple segments-up to the window limit-before waiting for further acknowledgments. Hard-coding a fixed buffer defeats that adaptive behavior.

The Nagle algorithm is intended to bundle small writes into larger segments to reduce overhead, not to match sender rate to receiver capacity. The urgent pointer signals out-of-band data but plays no role in routine flow control. Slow-start threshold (ssthresh) governs congestion control; it reacts to network congestion rather than receiver buffer availability.

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